1. Field
The present disclosure relates generally to carbon-based electrodes for energy storage devices, and more specifically to carbon-based electrodes that contain particles of one or more molecular sieve material and their related methods of fabrication.
2. Technical Background
Energy storage devices such as ultracapacitors may be used in a variety of applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Ultracapacitors also known as electrochemical double layer capacitors (EDLCs) have emerged as an alternative or compliment to batteries in applications that require high power, long shelf life, and/or long cycle life. Ultracapacitors typically comprise a porous separator and an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layers that are created at the interfaces between the electrodes and the electrolyte. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon that is incorporated into the electrodes.
Carbon-based electrodes suitable for incorporation into energy storage devices are known. Activated carbon is widely used as a porous material in ultracapacitors due to its large surface area, electronic conductivity, ionic capacitance, chemical stability, and/or low cost. Activated carbon can be made from synthetic precursor materials such as phenolic resins, or natural precursor materials such as coals or biomass. With both synthetic and natural precursors, the activated carbon can be formed by first carbonizing the precursor and then activating the intermediate product. The activation can comprise physical (e.g., steam) or chemical activation at elevated temperatures to increase the porosity and hence the surface area of the carbon. The carbon-based electrodes can include, in addition to activated carbon, a conductive carbon such as carbon black, and a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF). The activated carbon-containing layer (carbon mat) is typically laminated over a current collector to form the carbon-based electrode.
The choice of separator and electrode materials directly affect the performance of the device, including the achievable energy density and power density. The energy density (E) of an EDLC is given by E=½ CV2, where C is the capacitance and V is the device's operating voltage. Recently, engineered carbon materials have been developed to achieve higher capacitance. To achieve higher capacitance, activated carbon materials with high surface area (500-2500 m2/g) may be used.
A further approach to increasing the energy density is to increase the capacitor's operating voltage. In this regard, aqueous electrolytes have been used in EDLCs for lower voltage (<1V) operation, while organic electrolytes have been used for higher voltage (2.3-2.7 V) devices. However, to achieve even higher energy densities, there is a need to increase the voltage envelop from conventional values of about 2.7 V to around 3.0 V. Such an increase from 2.7 to 3.0 V will result in a 23% increase in the energy density.
Thus, in order to realize higher energy densities and higher power densities, next generation EDLCs will likely operate at high applied voltages. It may be desirable, therefore, to minimize unwanted Faradaic reactions between the activated carbon and the liquid electrolyte, particularly at the higher potentials.
Accordingly, it would be advantageous to minimize these Faradaic reactions by minimizing, for example, the water content in carbon-based electrodes and the attendant carbon materials for operation at high voltages. The activated carbon materials can possess a high surface area to volume ratio and minimal reactivity, particularly with the organic electrolyte at elevated voltages, and can be used to form carbon-based electrodes that enable efficient, long-life and high energy density devices.